This invention relates generally to communication networks or local area networks (LANs) and, more particularly, to the architecture of a network hub for the interconnection of multiple network components, such as multiport repeaters, bridges, switches and routers. Bridges, switches and routers are connected to more than one network and provide a convenient mechanism for transferring data packets between one network and another. Multiport repeaters perform a similar function, but operate at a bit level rather than a message packet level. Repeaters operate practically in "real time" to relay messages bit-by-bit from one port to other ports, while bridges and routers may store data packets temporarily, and perform an intelligent message routing function.
Even more particularly, the invention applies to networks that employ a protocol commonly referred to as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). One CSMA/CD design that is widely used in networks is known as Ethernet. The Ethernet design is specified by standards documents ISO 8802-3 and ANSI/IEEE 802.3, which will be referred to as the IEEE 802.3 standard. Although the mode of operation of networks under CSMA/CD rules is not directly pertinent to the present invention, a brief explanation follows by way of further background. Under the CSMA/CD rules for access to a network bus or cable, any station wishing to transmit must first "listen" to make sure that the cable is clear before beginning to transmit. All stations on the network have equal priority of access and may begin transmitting as soon as the line is clear and any required inter-packet delay has elapsed. However, if a first station that has started transmitting detects a "collision" with a transmission from another station, the first station continues transmitting for a short time to make sure that all stations wishing to transmit will detect the collision. Every other station detecting the collision also continues to transmit for a short time. Then each station that has detected a collision terminates transmission for some random period of time. The stations involved in the collision select random, and therefore usually different, delay times before trying transmission again.
A repeater is a device having multiple ports, each of which may be connected to single or multiple network stations. The IEEE 802.3 standard, in its Sections 9.1 to 9.8, sets forth a standard design of a repeater employing the CSMA/CD collision detection method. In one embodiment, a repeater is connected in a star configuration with one station connected to each port of the repeater. In an alternative embodiment, a port of the repeater may be connected to a plurality of stations by a transmission medium operating as a single Ethernet link. Thus, a repeater may have many more stations connected to it than there are ports on the repeater.
A repeater, in effect, logically connects the wires on its input ports such that all the devices connected to those wires are in the same network "collision domain." However, the repeater does not connect the ports by a simple electrical connection. Instead, the repeater listens to signals on each of the ports and repeats those signals on the other ports. One of the functions of a repeater is to detect collisions on one or more of its ports and to take appropriate action consistent with the IEEE 802.3 standard. For example, if a collision is detected on one port, when the incoming signal on that port indicates that more than one station is transmitting, the repeater must advise the stations on its other ports of the existence of a collision, by sending out a standard jamming signal through all the other ports. This type or collision is usually referred to as a "receive" collision. Another type of collision situation arises when a repeater receives data over two or more active ports simultaneously. The repeater is then aware of a collision, but none of the stations connected to the ports are aware of a collision, even the stations connected to the active ports from which the signals were received. This is called a "transmit" collision, and the repeater responds by sending jamming signals to all of its ports.
Repeaters provide a convenient mechanism for expanding networks. The IEEE 802.3 standard requires that the number of repeaters encountered by a transmitted message before a bridge or router is encountered, be limited in number. To meet this limitation but still provide for further network expansion, network designers use multiple "half-repeaters," each of which may have as many ports as a conventional repeater, but which are closely coupled together by an inter-repeater bus structure. The bus structure not only provides for the transmission of Ethernet data between two half-repeaters, but also transmits state information between the two modules, such that they cooperate to perform the function of a single repeater.
Half-repeaters are typically installed in a central wiring facility referred to as a hub, which includes a backplane of multiple conductors, i.e. a bus, into which the half-repeater modules are removably plugged. The interface between the bus and the half-repeater modules is standardized such that other network components, such as bridges, routers, and individual stations, may also be plugged into the bus. Since, multiple half-repeaters or other modules all have a need to transmit data and state information over a common bus, some form of arbitration must be provided to regulate bus usage. In general, there are two categories of bus arbitration schemes: central arbitration and distributed arbitration. Prior to the present invention, hub designs have included inter-repeater buses (IRBs) or intermodule buses (IMBs) having several parallel conductors. For example the IRB manufactured by AMD uses a centrally arbitrated bus scheme with four common bus pins and one additional request pin for each half-repeater. This requires six pins for two half-repeaters, seven pins for three half-repeaters, and so forth. An alternative intermodule Ethernet bus (IMB) scheme using distributed arbitration is defined in pending patent application Ser. No. 08/270,072, filed Jul. 1, 1994, entitled "Hot-Swappable Inter-Module Interconnect of Simultaneous Use with Distributed LAN Repeaters and Stations," assigned to the same assignee as the present invention and having attorney Docket No. 93-0393. This scheme allows up to fifteen half-repeaters or other Ethernet devices to communicate over six pins in the bus.
It will be appreciated from the foregoing that there is still a need for improvement in the design of network hubs for interconnecting half-repeaters and other network components. Ideally, the intermodule bus should be as simple as possible to minimize the size and cost of the interconnection architecture, but without detracting from its functionality. The present invention meets this goal.